1. Field of the Invention
The invention relates to a method and apparatus for selectively carrying first and second weight loads of a tubing string in a subterranean well, and for separating said tubing string through the apparatus when the second weight load is exceeded.
2. Description of the Prior Art
In the production of well fluids, such as oil and/or gas, from wells, it has been the practice to provide automatically closeable shut-off or safety valves which are located downhole in the well and are held open by control fluid pressure, the valves closing automatically when control fluid pressure is purposely reduced to allow the valves to close or damage occurs to the control fluid system at the well head or on an offshore platform. Such valves are employed below the well head, and in the case of offshore wells, the valves are installed below the mud line at such depth as may be desired or established by regulation, so that in the event of damage of the well caused by shifting earth or subsidence, or well head catastrophe, the well can be shut in to avoid loss of valuable well fluids into the water, and also, to avoid contamination of the water and the shore.
Many offshore wells are produced from spaced well zones through separate strings of production tubing, and a safety or shut-off valve is required for each zone. Since, from time-to-time, it is necessary to perform various remedial operations through the tubing strings, it is preferred that the safety valves be easily removed from the well for service or repair. Accordingly, commercially available safety or shut-off valves have been provided which have been run into the well casing on production tubing and landed in a tubing hanger which supports the greater weight of the downwardly extending production tubing strings. Typically, such a tubing hanger has been run into the well casing on a setting tool to a desired location, and, in the case of an offshore well, to a prescribed depth below the mud line. In such an apparatus, the tubing hanger is anchored in the well casing and the setting tool is released from the tubing hanger and removed from the well. The tubing hanger provides a seat for the safety or shut-off valve assembly which is run into the well on an upward extension of the production tubing and landed in the tubing hanger, subsequent to the setting of the hanger and retrieval of the hanger setting tool.
Typical of such prior art apparatuses is that as disclosed in U.S. Pat. No. 3,771,603, issued Nov. 13, 1973, entitled "Dual Safety Valve Method And Apparatus", to Talmadge L. Crowe, the disclosure of which is hereby incorporated herein by reference. The necessity of two trips into the hole with work strings and/or other means to first carry and anchoringly set the tubing hanger and thereafter land the conduits containing the safety valves therein is an economic deterrent since considerable rig time is expended in running a first work string and/or other means for anchoring the hanger, retrieving the work string and/or other means, and thereafter running the production tubing containing the safety valve or valves into sealing engagement with the hanger.
In co-pending application Ser. No. 36,964, filed May 7, 1979, entitled "Single Trip Tubing Hanger Assembly", there is disclosed a latch assembly used in conjunction with a tubing hanger carried on upper production tubing strings, the production strings typically carrying safety valves to control well production fluid transmission therethrough. The present invention is directed to a unique shear-out safety joint apparatus which, for example, may provide a means of separating the tubing conduit below the safety valve means when a predetermined load across the shear-out safety joint is exceeded, and also provides a bridge so that a weight load up to the full production tubing strength initially can be carried for a preliminary operation, such as the setting of a packer apparatus below the hanger assembly. Thereafter, the bridge may be removed and the load capability of the shear-out safety joint apparatus is relaxed.